1. Field of the Invention
The present invention relates to a planar antenna structure, and more particularly to a high gain broadband planar antenna.
2. Description of Related Art
In recent years, the blooming of wireless communications gives rise to our increasingly higher requirements for the bandwidth and data transmission rate of wireless communications. As to the wireless local area network (Wi-Fi), the data transmission rate is improved from 2 MB and 11 MB up to 54 MB, but the transmitting distance is still restricted to a range from one hundred to two hundred meters. If the transmitting distance is extended to several kilometers, the Wi-Fi will be unable to work, and thus starting the development of a new WiMAX communication technology. The WiMAX is a type of wide area network (WAN) communications having a transmitting range up to 30 miles and according with the IEEE 802.16 standard. Most of the communication ranges of the wireless local area network fall within one to two hundred meters and are belonged to short-distance transmission. However, the WiMAX provides data transmissions that cover a range from several kilometers to even tens of kilometers and transceiver radio through outdoor antennas stably. Therefore, the radio of WiMAX can be transmitted farther, and the IEEE 802.11 wireless local area network can only depend on its built-in transceiver antenna for its signal transmissions.
Regardless of the IEEE 802.11a/g or WiMAX specification, the antenna used for transmitting/receiving signals through the IEEE 802.11a/g or WiMAX becomes one of the important components in the wireless communication field. At present, most manufacturers favor the use of printed circuit boards for the production of the antenna, since the printed circuit board has the advantages of an easy manufacturing process and a low cost.
Many technology related to antenna designs for dual-frequency operation have been disclosed. Referring to FIG. 1 for the schematic view of the dual-band dipole antenna structure, the antenna structure includes a signal terminal 1 and a ground terminal 2 of a same shaped antenna, and both signal terminal 1 and ground terminal 2 are disposed on a PCB substrate 3 and a conical feeder 4 is connected separately to the signal terminal 1 and the ground terminal 2 to define a broadband dual-frequency dipole antenna structure, wherein the signal terminal 11 and the ground terminal 12 are bent into a U-shape.
Referring to FIG. 2 for the multi-frequency printed dipole antenna, the antenna includes a lengthwise insulating substrate 5, a first duality (radiating elements) 61a, 61b, a second duality (radiating elements) 62a, 62b, a third duality (radiating elements) 63a, 63b, a first pair of connecting parts 64a, 64b, a second pair of connecting parts 65a, 65b, a connecting plate 66, a feeder 7, and a capacitor 8.
Most of the present improved antenna structures can achieve good radiation efficiency and antenna gain in the operating bandwidth of the IEEE 802.11a/b/g, but the foregoing prior arts cannot satisfy the high gain requirement of a broadband (3.3˜3.8 GHz) required for the WiMAX technology, and the antenna gain of these prior arts is only in the range of 1.8˜2 dBi. Therefore, finding a high gain broadband planar antenna in compliance with the frequency of the IEEE 802.11/a/b/g and WiMAX is a subject that demands R&D engineers' immediate attention.